1. Field of the Invention
The present invention relates to a suspension of a vehicle such as an automobile, and more particularly, to an active suspension of a vehicle.
2. Description of the Prior Art
A type of hydraulic active suspension of a vehicle such as an automobile is known, as described in, for example, Japanese Patent Laid-open Publication 2-27406, which comprises hydraulic actuators each provided for each vehicle wheel so as to increase or decrease the height of the vehicle body at a portion thereof corresponding to each vehicle wheel according to supply or exhaust of working fluid thereto or therefrom, control valves each controlling the supply or exhaust of the working fluid to or from each hydraulic actuator, an acceleration detection means for detecting acceleration of the vehicle, and a control means for controlling the control valves based upon a control parameter related with the acceleration of the vehicle, wherein the pressure in each actuator is controlled, in an aspect thereof, according to a parameter proportional to a lateral acceleration of the vehicle body when the vehicle makes a turn so that the rolling of the vehicle body is suppressed.
By such an active suspension it is possible to suppress the rolling of the vehicle body during a turn of the vehicle, as the pressure in each actuator is controlled according to the parameter proportional to the lateral acceleration of the vehicle body.
However, when a vertical force is exerted to a vehicle body during a turn of a vehicle due to a cornering force at a vehicle wheel, the vehicle body may heave up or dive down according to such a vertical force. This phenomenon will be seen in both the passive and active suspensions. However, in the active suspension which operates to suppress the rolling of the vehicle body by a control of the pressure in the hydraulic actuator, the phenomenon becomes more noticeable.
FIG. 14 is an illustration to explain the above-mentioned phenomenon.
Referring to FIG. 14, denoting the mass of a vehicle body 200 by M, a lateral acceleration of the vehicle body by Gy, a lateral force at an outside-in-turn vehicle wheel 202out by Fyout, and a lateral force at an inside-in-turn vehicle wheel 202in by Fyin, respectively, the following equilibrium of force in the lateral orientation is obtained: EQU M.multidot.Gy=Fyout+Fyin (1)
Further, denoting a ground contact force at the outside-in-turn vehicle wheel 202out by Fzout, a ground contact force at the inside-in-turn vehicle wheel 202in by Fzin, and the acceleration of gravity by Gz, respectively, the following equilibrium of force in the vertical orientation is obtained: EQU M.multidot.Gz=Fzout+Fzin (2)
Further, denoting the height of the mass center C of the vehicle body 200 by H, and the tread by T, respectively, the following equilibrium of the roll moment is obtained: EQU M.multidot.Gy.multidot.H=(Fzout-Fzin).multidot.T/2 (3) EQU Fzout-Fzin=2M.multidot.Gy.multidot.H/T (3')
Theoretically, the ratio of the lateral force Fy (i.e. Fyout or Fyin) to the ground contact force Fz (i.e., Fzout or Fzin) acting at a vehicle wheel is a function of the slip angle of the vehicle wheel or its tire. However, in fact, when the turning rate of the vehicle increases, said ratio is deformed as shown by a solid curve in FIG. 15. Therefore, denoting said function of the slip angle as Q(s), the cornering force performance is approximated to, with certain constants a and b, as follows: EQU Fy=Q(s).multidot.(a.multidot.Fz-b.multidot.Fz.sup.2) (4)
Now, in the suspension structure shown in FIG. 14, in which the vehicle wheel 202 is connected with the vehicle body 200 by a double wishbone type suspension link including a lower arm 204 and an upper arm 206, and a hydraulic actuator 208 for the active suspension is connected between the lower arm 204 and the vehicle body 200, the point of intersection, K, of the axis of the outside-in-turn lower arm 204out and the axis of the inside-in-turn lower arm 204in may be considered as a point of action of force at which the side forces acting at the outside-in-turn and inside-in-turn vehicle wheels during a turning of the vehicle, i.e. the cornering forces, exert a vertical force to the vehicle body so as to cause it heave up or dive down, by overcoming the suspension forces exerted by the actuators.
Denoting the angle which the point K expands with the ground contact point of the vehicle wheel relative to the ground surface by .theta. and assuming that this angle is substantially the same with respect to the opposite sides vehicle wheels, a vertical force which is exerted to the vehicle body at point K due to the cornering forces at the outside-in-turn and inside-in-turn vehicle wheels is expressed by: EQU (Fyout-Fyin)tan.theta.
When the ratio of the distance between an inboard end of the lower arm 204 pivotably connected with the vehicle body 200 and a pivot joint of the lower arm 204 and a lower end of the actuator 208 to the distance between the inboard pivot joint end and an outboard end of the lower arm 204 pivotably connected with a wheel carrier is denoted by Ra, the above-mentioned vertical force affects the operation of the actuators 208 under the lever ratio Ra as follows: EQU Fj=(Fyout-Fyin)(tan.theta.)/Ra (5)
This vertical force Fj will be called herein "jackup force". From the condition according to equation (4), EQU Fyout=Q(s).multidot.(a.multidot.Fzout-b.multidot.Fzout.sup.2)(6) EQU Fyin=Q(s).multidot.(a.multidot.Fzin-b.multidot.Fin.sup.2) (7)
From equations (6) and (7), ##EQU1## From equations (1), (6) and (7), EQU M.multidot.Gy=Q(s).multidot.{a.multidot.(Fzout+Fzin)-b.multidot.(Fzout.sup. 2 +Fin.sup.2)}
Therefore, EQU Q(s)=M.multidot.Gy/{a.multidot.(Fzout+Fzin)-b.multidot.(Fzout.sup.2 +Fin.sup.2)}
Therefore, EQU Fyout-Fyin=M.multidot.Gy.multidot.(Fzout-Fzin){a-b.multidot.(Fzout+Fzin)/{a .multidot.(Fzout+Fzin)}b.multidot.(Fzout.sup.2 +Fin.sup.2)}(9)
From equations (2) and (3), EQU Fzout=M.multidot.Gz/2+M.multidot.Gy.multidot.H/T EQU Fzin=M.multidot.Gz/2-M.multidot.Gy.multidot.H/T
Therefore, EQU Fzout.sup.2 +Fzin.sup.2 =M.sup.2 .multidot.Gz.sup.2 /2+2M.sup.2 .multidot.Gy.sup.2 .multidot.H.sup.2 /T.sup.2 ( 10)
By substituting equations (2), (3') and (10) for equation (9), EQU Fyout-Fyin={2M.multidot.H.multidot.(a-b.multidot.M.multidot.Gz)/T}.multidot .Gy.sup.2 /{(a.multidot.Gz-b.multidot.M.multidot.Gz.sup.2 /2)-(2b.multidot.M.multidot.H.sup.2 /T.sup.2).multidot.Gy.sup.2 }
Therefore, EQU Fj={2M.multidot.H.multidot.(a-b.multidot.M.multidot.Gz)/T}.multidot.(tan.th eta.).multidot.Gy.sup.2 /{(a.multidot.Gz-b.multidot.M.multidot.Gz.sup.2 /2)-(2b.multidot.M.multidot.H.sup.2 /T.sup.2).multidot.Gy.sup.2 }/Ra
Substituting u for {2M.multidot.H.multidot.(a-b.multidot.M.multidot.Gz)/T}.multidot.tan.theta ., v for (a.multidot.Gz-b.multidot.M.multidot.Gz.sup.2 /2).multidot.Ra, and w for (2b.multidot.M.multidot.H.sup.2 /T.sup.2).multidot.Ra, EQU Fj=u.multidot.Gy.sup.2 /(v-w.multidot.Gy.sup.2)
Since in equation (4) "b" is generally very small as compared with "a", the general performance of the jackup force Fj due to the lateral acceleration Gy is such as shown in FIG. 16.
As will be seen from FIG. 16, the jackup force Fj is substantially zero when the absolute value of the lateral acceleration Gy is less than a certain moderate value Gy1, but increases gradually when the lateral acceleration increases beyond Gy1.
According to each particular geometry of the suspension mechanism like the double wishbone type link in the example shown in FIG. 14, the value of Fj becomes positive or negative for an increase of the lateral acceleration in the same direction.